What it means and why
The dioptre, written D, is a unit of optical power. In a simplified thin-lens model, power equals the reciprocal of focal length in metres. A positive sign represents convergence and a negative sign divergence in this convention.
Do not confuse D with degrees used for an axis, millimetres used for fitting, or the score on a vision chart. These units answer different questions and cannot be exchanged by moving a decimal point.
A worked example
Teaching optics, not a prescription: a +2.00 D thin lens has a 0.50 m focal length for parallel light in the simplified model. It does not follow that every person wearing +2.00 glasses should read at 50 cm, because the wearer’s eye and other prescription components are missing from that model.
What it does not tell you
A dioptre value does not predict visual-acuity lines. A prism dioptre, written Δ, measures a different effect and is not interchangeable with spherical D.
THINK IT THROUGHCan 180° be compared numerically with 2.00 D?
No. One is an orientation angle; the other is optical power. First identify the field and unit.
Sources & reading notes
Page updated:
- ABDO · Explaining spectacle prescriptions, part 1 ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.